TSH (thyroid-stimulating hormone)

A pituitary signal that rises when your thyroid is underactive and falls when it’s overactive — a sensitive but indirect gauge of thyroid function.

What it measures

The blood level of thyrotropin (TSH). Because of the feedback loop, it moves in the opposite direction to your thyroid hormone.

Why it matters

It’s the most sensitive first-line screen for thyroid problems: a high TSH suggests an underactive thyroid, a low TSH an overactive one.

Healthy target

~0.4–4.0 mIU/L (typical adult lab range)

How to read your value

One value isn’t a diagnosis — TSH fluctuates with time of day, illness and normal biological variation. A mildly abnormal result should be repeated and, when needed, paired with free T4 and your symptoms before any label. The popular “optimal under 2.5” idea isn’t supported for treating people who feel well.

What can affect it

• Time of day and acute illness
• Pregnancy
• Biotin supplements (can interfere with the assay)
• Certain medications and the specific lab assay

Does the target differ by age or sex?

The healthy upper limit drifts up with age — values a little above 4 can be normal ageing in older adults, so using a young-adult cut-off there risks overdiagnosis.

When to talk to a clinician

Persistently out-of-range values on repeat testing, or thyroid symptoms, warrant confirmation with free T4 and a clinical assessment.